Insulating paperboard

ABSTRACT

An insulating paperboard contains at least one layer of cellulose fibers. The one layer is at least partially composed of cellulosic fibers and synthetic fibers. The paperboard provides sufficient insulation to provide a hot water AT across the paperboard of at least 0.6° C. per 0.1 mm of caliper change. A hot cup may be produced from the insulating paperboard.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.11/70,868 filed Jun. 30, 2005, priority from the filing date of which isherby claimed under 35 U.S.C. § 120.

FIELD

The present application relates to an insulating paperboard, and moreparticularly to an insulating paperboard containing cellulosic fibersand synthetic fibers and mixtures of cellulosic fibers, processedcellulosic fibers and synthetic fibers.

BACKGROUND

Hot foods, particularly hot liquids, are commonly served and consumed indisposable containers. These containers are made from a variety ofmaterials including paperboard and foamed polymeric sheet material. Oneof the least expensive sources of paperboard material is cellulosefibers. Cellulose fibers are employed to produce excellent paperboardsfor the production of hot cups, press-molded paperboard plates, andother food and beverage containers. Conventional paperboard producedfrom cellulosic fibers, however, is relatively dense, and therefore,transmits heat more readily than, for example, foamed polymeric sheetmaterial. Thus, hot liquids are typically served in doubled cups ofconventional paperboard or in cups with sleeves.

It is desirable to possess an insulating paperboard produced fromcellulosic material and synthetic fibers that has good insulatingcharacteristics, that will allow the user to sense that food in thecontainer is warm or hot and at the same time will allow the consumer ofthe food or beverage in the container to hold the container for alengthy period of time without the sensation of excessive temperature.It is further desirable to provide an insulating paperboard that can betailored to provide a variety of insulating characteristics so that thetemperature drop across the paperboard can be adjusted for a particularend use.

BRIEF DESCRIPTION OF THE DRAWINGS

This application will become more readily appreciated and understood byreference to the following detailed description, when taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of a two-ply paperboard whichcan be constructed in accordance with the present application;

FIG. 2 is an isometric view of a hot cup made from the paperboardsimilar to that shown in FIG. 1 with a portion cut away; and

FIG. 3 is an enlarged cross-sectional view of a portion of thepaperboard used to make the hot cup shown in FIG. 2.

DETAILED DESCRIPTION

Referring to FIG. 1, the substrate 10 for the insulating paperboard 12of the present application is produced in a conventional manner fromreadily available fibers such as cellulosic fibers and synthetic fibersor cellulosic fibers, processed cellulosic fibers and synthetic fibers.The paperboard of the present application can be made in a single-ply, atwo-ply construction, or a multi-ply construction, as desired.

The distinguishing characteristic of the present application is that atleast one ply, 14, of the insulating paperboard, whether a single-ply ora multiple-ply structure, contains cellulosic fibers in addition tosynthetic fibers. Processed cellulosic fibers may also be included. Thecellulosic fibers and synthetic fibers increase the insulatingcharacteristics of the board when those fibers create a lower densitystructure than otherwise. As defined herein cellulosic fibers (alsocalled chemical pulp fibers) useable in the present application arederived primarily from wood pulp. Suitable wood pulp fibers for use withthe application can be obtained from well-known chemical processes suchas the kraft and sulfite processes, with or without subsequentbleaching. Softwoods and hardwoods can be used. Details of the selectionof wood pulp fibers are well known to those skilled in the art. Forexample, suitable cellulosic fibers produced from southern pine that areuseable in the present application are available from a number ofcompanies including Weyerhaeuser Company under the designations CF416,PL416, FR416 and NB416. A bleached Kraft wet lap pulp, manufactured byWeyerhaeuser, Federal Way Wash., KKT, Prince Albert Softwood and GrandePrairie Softwood are examples of northern softwoods that can be used. Asused herein, processed cellulosic fibers include fibers that arechemically processed to change the cellulose from Cellulose I toCellulose II, such as mercerized and mercerized flash dried fibers inwhich the mercerization is conducted as one stage in the bleachingprocess, Mercerized fibers such as HPZ and mercerized flash dried pulpsuch as HPZ III, both manufactured by Buckeye Technologies, MemphisTenn. and Porosinier-J-HP available from Rayonier Performance FibersDivision, Jessup, Ga. are suitable for use in the present application.These mercerized softwood pulps have an α-cellulose purity of 95% orgreater and are stiff fibers. Processed fibers also include mechanicallyaid chemimechanically treated fibers. such as chemithermomechanical pulpfibers (CTMP), bleached chemithermomechanical pulp fibers (BCTMP),thermomechanical pulp fibers (TMP), refiner groundwood pulp fibers andgroundwood pulp fibers. Recycled or secondary wood pulp fibers are alsosuitable.

Examples of these pulps are TMP (thermomechanical pulp) made by Bowater,Greenville, S.C., a TMP (thermomechanical pulp) made by Weyerhaeuser,Federal Way, Wash., made by passing wood chips trough three stages ofdual refiners and a CTMP (chemithermomechanical pulp) obtained fromNORPAC, Longview, Wash., sold as a CTMP NORPAC Newsprint Grade. Otherprocessed fibers include specially dried jet dried fibers and treatedjet dried cellulosic fibers manufactured by the Weyerhaeuser Company bythe method described in U.S. application Ser. No.10/923,447 filed Aug.20, 2004. In this method a slurry of pulp fibers is dewatered to aconsistency of approximately 34% and then passed through a jet drierhaving an inlet temperature of approximately 190° C. to 400° C. anoutlet temperature of 50° C. to 205° C. and a steam pressure ofapproximately 1082 kPa (157 psig). Fibers made by this process havekink, twist and curl. Additional processed fibers include flash driedand treated flash dried fibers as described in U.S. Pat. No. 6,837,970,Mixtures of processed cellulosic fibers and synthetic fibers can also beused as well as mixtures of processed fibers, cellulosic fibers andsynthetic fibers.

Synthetic fibers can include, polymeric fibers, such as polyolefin,polyamide, polyester, polyvinyl alcohol, and polyvinyl acetate fibers.Suitable polyolefin fibers include polyethylene and polypropylenefibers. Suitable polyester fibers include polyethylene terephthalatefibers. Other suitable synthetic fibers include, for example, acrylics,nylon and rayon fibers. Crimped synthetic fibers, multi-lobal orbicomponent fibers may also be used.

Paperboard of the present application may have a broad set ofcharacteristics. For example, in one embodiment its basis weight canrange from 200 gsm to 500 gsm, in another embodiment the basis weightranges from 250 gsm to 400 gsm. In another embodiment the basis weightof the paperboard is equal to or greater than 250 gsm. In one embodimentthe insulating paperboard has a density of less than 0.5 g/cc, in yetanother embodiment the density is from 0.3 g/cc to 0.45 g/cc, and inanother embodiment the density is from 0.35 g/cc to 0.40 g/cc.

When at least one ply of the paperboard contains cellulosic fibers andsynthetic fibers in accordance with the present application,advantageous temperature drop characteristics can be achieved. Thesetemperature drop characteristics can be achieved by altering the amountof cellulosic fibers and synthetic fiber introduced into the paperboard,by adjusting the basis weight of the paperboard, by adjusting thecaliper of the paperboard after it has been produced by running it, forexample, through nip rolls, and of course, by varying the number andthickness of additional plies incorporated in the paperboard structure.The insulating paperboard properties are given in Table 1 below. Sampleswere made according to Example 1. TABLE 1 Insulating PaperboardProperties Basis Taber Tensile Wt Density, Caliper Stiffness Index ZDTFiber Wt. % (gsm) (g/cc) (mm) (g-cm) (Nm/g) (kPa) ΔT, ° C. T-255 5 2230.56 0.40 70.2 61.9 601 3.6 Crimped Polyester 60 238 0.44 0.53 47.9 20.7168 5.1 T-255 5 576 0.50 1.16 1068.8 38.8 513 6.6 Crimped Polyester 60430 0.55 0.79 161.4 22.4 194 6.9 Crimped Acrylic 60 328 0.34 0.98 157.426.1 106 7.8 Crimped Polyester 60 603 0.58 1.04 401.3 20.5 128 11.4

In one embodiment the paperboard has a caliper greater than or equal to0.4 mm, a basis weight equal to or greater than 200 gsm, and a densityless than 0.5 g/cc In another embodiment the paperboard of the presentapplication exhibits a hot water AT of at least 4.8° C. at a caliper of0.5 mm and a hot water ΔT of 8.1° C. at a caliper of at least 1 mm. Therelationship of hot water ΔT (as defined below) to caliper is a linearone between the calipers of 0.4 mm and 1 mm and continues to be linearwith a reduction in the caliper below 0.4 mm or an increase above 1 mm.Stated another way, a paperboard constructed in accordance with thepresent application having a caliper of 0.4 mm or greater will exhibit ahot water ΔT of about 0.6° C. per 0.1 mm of caliper change. The abovetemperature values are based on a linear regression equation of calipervs. ΔT using the data in Table 1 and based on the coefficients in Table2. The statistical parameters are given in Table 2. TABLE 2 RegressionStatistics Multiple R 0.74 R Square 0.54 Observations 6 CoefficientsLower 95%* Upper 95%* Intercept 1.56 −5.61 8.72 X Variable 6.55 ˜1.7914.88*Confidence limit

Using the coefficients established in the regression equation above, arelationship can be established for the ΔT at different caliper levels.

Mixtures of cellulosic fibers, processed cellulosic fibers and syntheticfibers can also be used. In one embodiment, when 50/50 mixtures ofprocessed cellulosic fibers and synthetic fibers are used in thepaperboard the paperboard of the present application exhibits a hotwater ΔT of at least 4.5° C. at a caliper of 0.5 mm and a hot water ΔTof 8.7° C. at a caliper of at least 1 mm. The relationship of hot waterΔT to thickness is a linear one between the calipers of 0.4 mm and 1 mmand continues to be linear with a reduction in the caliper below 0.4 mmor an increase above 1 mm. Stated another way, a paperboard constructedin accordance with the present application having a caliper of 0.5 mm orgreater will exhibit a hot water ΔT (as defined below)

of about 0.8° C. per 0.1 mm of caliper change. These temperature valuesare based on a linear regression equation of caliper vs. ΔT, using thecoefficients in Table 4. Paperboard properties are shown in Table 3;samples of the paperboard were made according to Example 2. TABLE 3Insulating Paperboard Properties Basis Taber Tensile Wt Bulk CaliperStiffness Index ZDT Fiber Wt. % (gsm) (g/cm3) (mm) (g-cm) (Nm/g) (kPa)ΔT, ° C. 50/50 HPZ III/T255 5 226 1.76 0.40 77.2 66.6 612 2.4 50/50HPZ/T105 30 230 2.21 0.51 79.6 47.7 413 5.4 50/50 HPZ III/T255 60 2243.33 0.75 68.7 20.8 114 9.2 50/50 HPZ III/T255 5 579 1.92 1.11 1097.043.2 537 6.0 50/50 HPZ/T105 30 558 2.23 1.25 861.8 33.7 228 11.6 50/50HPZ III/T255 60 577 2.69 1.56 511.8 15.7 108. 14.0

TABLE 4 Regression Statistics Multiple R 0.86 R Square 0.75 CoefficientsLower 95.0%* Upper 95.0%* Intercept 0.45 −6.3 7.2 X Variable 8.23 1.614.9*Confidence Limit

The paperboard of the application can be a single-ply product. When asingle-ply product is employed, the density characteristics of thepaperboard of the present application allows the manufacture of athicker paperboard at a reasonable basis weight. To achieve the sameinsulating characteristics with a normal paperboard, the normalpaperboard thickness would have to be doubled relative to that of thepresent application. Using cellulosic fibers, processed cellulosicfibers together with synthetic fibers, all insulating paperboard havingthe same basis weight as a normal paperboard can be made. Thiseffectively allows the manufacture of insulating paperboard on existingpaperboard machines with minor modifications and minor losses inproductivity. Moreover, a one-ply paperboard has the advantage that thewhole structure is at an acceptable density. Alternatively, thepaperboard of the application can be multi-ply product, and include two,three, or more plies. Paperboard that includes more than a single-plycan be made by combining the plies either before or after drying.Multi-ply paperboard can be made by using multiple headboxes arrangedsequentially in a wet-forming process, or by a baffled headbox havingthe capacity of receiving and then laying multiple pulp furnishes. Theindividual plies of a multi-ply product can be the same or different.

The paperboard of the present application can be formed usingconventional papermaking machines including, for example, Rotoformer,Fourdrinier, inclined wire Delta former, and twin wire forming machines.

In one embodiment when a single-ply paperboard is used in accordancewith the present application, it is homogeneous in composition. Thesingle ply, however, may be stratified wit respect to composition andhave one stratum enriched with cellulosic fibers and synthetic fibersand another stratum enriched with celluosic fibers to provide a smooth,denser, less porous surface. In another embodiment, one stratum may beenriched with cellulosic fibers, processed cellulosic fibers andsynthetic fibers and another stratum enriched with celluosic fibers toprovide a smooth, denser, less porous surface.

It is most economical to produce a paperboard that is homogeneous incomposition where the cellulosic fibers and synthetic fibers orcellulosic fibers, processed cellulosic fibers and synthetic fibers areuniformly intermixed. In one embodiment cellulosic fibers are present inthe insulating ply or layer in an amount from about 40% to about 95% andthe synthetic fibers are present in an amount of from 5% to 60% by totaldry weight. In a two ply structure, for example, the first ply maycontain 100% cellulosic fibers while the second ply may contain from 40%to 95% cellulosic fibers and 5% to 60% synthetic fiber. In anotherembodiment the processed cellulosic fiber and synthetic fiber, whereeach are present in an equal amount, (50/50) may be present in an amountof from 10% to 60% by total dry weight of the fiber. The balance of thefibers are cellulosic fibers.

The paperboard of the present application has a broad set of strengthproperties when cellulosic fibers and synthetic fibers are used. Forexample, in one embodiment the Taber stiffness may range from about 50g-cm to about 1100 g-cm. In another embodiment the Taber stiffnessranges from about 150 to about 600 g-cm. Taber stiffness was determinedby ISO 24393: 1992 E except for units reported, the TAPPI counterpart is489 OM-92.

The paperboard with cellulosic fibers and synthetic fibers also has arange of tensile properties which can be tailored. In one embodiment thetensile index ranges from about 20 Nm/g to about 70 Nm/g. In anotherembodiment the tensile index ranges from about 30 Nm/g to about 50 Nm/g.Tensile index was determined by TAPPI 494.

In converting operations of a conventional board to a cup, it isestimated that a minimum Z-direction tensile (ZDT) of 275 kPa isnecessary for proper rim and top curl formation so that delaminationdoes not occur during this process. It is believed that with the presentboard the lower range can be extended to approximately 100 kPa. In oneembodiment ZDT (Z-Direction Tensile) ranges from about 100 kPa to about650 kPa, in another embodiment the ZDT ranges from about 300 kPa toabout 500 kPa. ZDT was determined by TAPPI 541.

In one embodiment when cellulosic fibers, processed cellulosic fibersand synthetic fibers are used in a mixture, Table 3, the Taber stiffnessmay range from about 60 g-cm to about 1100 g-cm. In another embodimentthe Taber stiffness ranges from about 200 to about 900 g-cm. Taberstiffness was determined by ISO 24393:1992 E except for units reported,the TAPPI counterpart is 489 OM-92.

The paperboard with cellulosic fibers, synthetic fibers and processedcelluosic fibers also has a range of tensile properties. In oneembodiment the tensile index ranges from about 15 Nm/g to about 70 Nm/g.In another embodiment the tensile index ranges from about 30 Nm/g toabout 50 Nm/g. Tensile index was determined by TAPPI 494.

In one embodiment ZDT (Z-Direction Tensile) ranges from about 100 kPa to650 kPa, in another embodiment the ZDT ranges from about 300 kPa toabout 500 kPa. ZDT was determined by TAPPI 541.

Sheet bulk was determined by TAPPI 411 and sheet density was calculatedas the reciprocal of sheet bulk.

The paperboard of the present application can be utilized to make avariety of structures, particularly containers, in which it is desiredto have insulating characteristics. Referring to FIG. 2, one of the mostcommon of these containers is the ubiquitous hot cup utilized for hotbeverages such as coffee, tea, and the like. Other insulating containersto such as the ordinary paper plate can also incorporate the paperboardof the present application. Also, carry-out containers conventionallyproduced of paperboard or of foam material can also employ thepaperboard of the present application. As shown in FIGS. 2 and 3, a hotcup type container produced in accordance with the present applicationmay comprise one or more plies 22 and 24, one of which, in thisinstance, 24, contains cellulosic fibers and synthetic fibers. In thisembodiment the cellulosic fibers and synthetic fibers are in theinterior ply 24. A liquid impervious backing 26 is preferably laminatedto the interior ply. The backing may comprise, for example, a variety ofthermoplastic materials, such as polyethylene. A similar constructioncan be made with cellulosic fibers, processed cellulosic fibers andsynthetic fibers. It is preferred that the paperboard used in the bottomof the cup contain no processed cellulosic fibers.

In addition to fibrous materials, the paperboard of the application mayinclude a binding agent. Suitable binding agents are soluble in,dispersible in, or form a suspension in water. Suitable binding agentsinclude those agents commonly used in the paper industry to impart wetand dry tensile and tearing strength to such products. Suitable wetstrength agents include cationic modified starch havingnitrogen-containing groups (e.g., amino groups), such as those availablefrom National Starch and Chemical Corp., Bridgewater, N.J.; latex; wetstrength resins, such as polyamide-epichlorohydrin resin (e.g., KYMENE557LX, Hercules. Inc., Wilmington, Del.), and polyacrylamide resin (see,e.g., U.S. Pat. No. 3,556,932 and also the commercially availablepolyacrylamide marketed by American Cyanamid Co., Stanford, Conn., underthe trade name PAREZ 631 NC); urea formaldehyde and melamineformaldehyde resins; and polyethylenimine resins. A general discussionon wet strength resins utilized in the paper field, and generallyapplicable in the present application, can be found in TAPPI monographseries No. 29, “Wet Strength in Paper and Paperboard”, TechnicalAssociation of the Pulp and Paper Industry (New York, 1965).

Other suitable binding agents include starch, modified starch, polyvinylalcohol, polyvinyl acetate, polyethylene/acrylic acid copolymer, acrylicacid polymers, polyacrylate, polyacrylamide, polyamine, guar gum,oxidized polyethylene, polyvinyl chloride, polyvinyl chloride/acrylicacid copolymers, acrylonitrile/butadiene/styrene copolymers, andpolyacrylonitrile. Many of these will be formed into latex polymers fordispersion or suspension in water.

Hot Water ΔT Test Procedure

Paperboard thermal performance is determined in a test unit that modelsthe heat transfer through a paper cup. A box of plexiglass measuring10×10×10 cm interior dimensions has a sample opening of 8.2 cm by 8.2 cmin one side. A gasket of surgical tubing is attached to the box aroundthe perimeter of the 8.2 cm×8.2 cm opening. A 10 cm×10 cm sample ofpaperboard is laminated on one surface with 10 cm wide 3 M Tartan 3765packaging tape. Alternatively, polyethylene may be extruded onto thesurface of the board. The paperboard sample is mounted onto theapparatus covering the sample opening with the sealed face toward theinterior. A separate piece of plexiglass (with the same outsidedimensions as the box and a hole 8.2 cm×8.2 cm cut out) is clamped overthe paperboard sample to hold it firmly against the box. The box isfilled with hot water at a temperature of 96.1° C. (205° F.) through asmall opening in the top of the box so that the water is in full contactwith the sample. A small stir bar is inserted into the box and theassembly is then placed on a stir plate to permit stirring during themeasurement phase. A K type thermocouple is inserted into the hot waterthrough the small opening in the box top and an infra-red thermometerIRCON Inc. Modline Series 3400 Radiation Thermometer, set to measure at0.96 emissivity is aimed at the outside center of the paperboard sampleat a 29.7cm distance and the IR radiation measured. A data logger,(HP34970A Data Acquisition/Switch Unit capturing the mVdc response fromthe radiation thermometer adjusted by a gain of 30.0 and an offset of100 and the mVdc response from the thermocouple but does not adjust it)records the temperature of both the inside water (from the thermocoupleinserted into the water) and the outside surface of the sample (from theinfrared radiation thermometer gun) from which the temperature drop (ΔT)can be calculated. When the water temperature reaches 85° C. (185° F.),the data capture is halted. The difference between the inside watertemperature and the outside paperboard temperature is calculated foreach data point captured by the data logger. A linear regressionanalysis is performed on the data for ΔT (inside water temperature minusoutside wall temperature) versus inside water temperature and, from theregression, the ΔT at 87.8° C. (190° F.) is determined. The linearregression analysis is run from the point of maximum outside walltemperature to a point on the curve that corresponds to an internalwater temperature of 85° C. (185° F.). ΔT is the difference intemperature between the water temperature of 87.8° C. (190° F.) and thecorresponding outside wall temperature of the paperboard on the testunit.

The hand sheet samples shown in Table 1 were prepared according themethod in Example 1. Table 3 represents samples made with mixtures ofcellulosic fibers, processed cellulosic fibers and synthetic fibers andwere made according to the method in Example 2.

EXAMPLE 1

This method is representative of making a 300 gsm board with 60% crimpedacrylic fiber. Other paperboards, shown in Table 1, of various basisweights and cellulosic and synthetic fiber levels can be made with theappropriate amounts and weights of fiber and other additives. Allpaperboard samples contained 5 percent by total dry fiber weightbleached Douglas Fir refined to 50 CSF (crill). The remainder of thecellulosic fiber (bleached Douglas fir) was refined to 510 CSF.Synthetic fibers used in different paperboards were as follows, 12.7 mm,6 denier crimped polyester fiber and 6.4 mm 1.5 denier crimped acrylicfiber were obtained from MiniFibers. Johnson City, Tenn. T-255 Celbond,a 6.4 mm, 2 denier polyester/polyethylene sheath/core fiber was obtainedfrom Invista, Witchita, Kans.

Crimped acrylic fiber, 18.4 air dried g fiber (98.4% consistency), 37.4g Douglas Fir refined to 510 CSF (29.1% consistency), 60.5 air dried gDouglas Fir refined to 50 CSF (2.5% consistency), (crill), and 3.02 gpolyvinylalcohol (Celvol 165SF PVOH, available from Celanese, DallasTex.), 100% solids, were disintegrated for 5 minutes in a BritishDisintegrator. The mixture was diluted to 4 L with deionized water andadjusted to a pH of 7.2-7.4 using NaHCO₃. The equivalent of 1 g/kg (2Lb/T) Kymene and 0.13 g/kg (0.26 lb/T) of Perform-PC8138 (both availablefrom Hercules, Wilmington, Del.) were added from 1% solutions each, andmixed for 2 minutes. AKD (alkyl ketene dimmer) available from Hercules,Inc., Wilmington Del. at 2 kg (4 lb/T) and 4.25 g/kg (8.5 lb/Ton) starch(Sta-Lok 300, available from Tate-Lyle, Decatur Ill.) were each addedand the mixture stirred for two (minutes. A 31.75×31.75 cm forming wire(155 mesh) was placed in the bottom of a Noble & Wood 12″ by 12″handsheet mold, the slurry poured into the sheet mold, diluted to 35liters with deionized water and mixed with a plunger. The slurry wasthen drained, dewatered by using blotters with even hand pressing untilthe sheet reached a consistency of approximately 20%. The sheet wasremoved from the screen and blotted further to approximately 30% solids.Blotters were placed on each side of the sample, the sample placedbetween damp felts and then passed through a press at 137.8 kPa (20 psi)to further dewater the sample. The solids content at this point wasapproximately 40%. The resulting sheet was placed on a drum dryer,(surface temperature of 121° C.), between two dry blotters and allowedto dry for 10 minutes. The sample was then inverted and allowed to dryan additional 10 minutes. The sample was conditioned in a 50% RelativeHumidity room for a minimum of 4 hours prior to testing.

EXAMPLE 2

This method is representative of making a 200 gsm board with 60% of a50/50 mixture of HPZ and T55 fiber. Other paperboards, shown in Table 3,of various basis weights, cellulosic fiber, processed cellulosic fiberand synthetic fiber levels can be made with adjustment to theappropriate amounts and weights of fiber and other additives. Allpaperboard samples contained 5 percent by total dry fiber weight DouglasFir refined to 50 CSF (crill). The remainder of the cellulosic fiber(bleached Douglas fir) was refined to 510 CSF.

HPZ, 6.59 g air dried g fiber (91.9% consistency), 6.13 air dried g T25524 (98.9% consistency), 24.29 air dried g Douglas Fir refined to 510 CSF(29 1% consistency), 40.4 air dried g Douglas Fir refined to 50 CSF(2.5% consistency), (crill), and 3.02 g polyvinylalcohol (Celvol 165SFPVOH, available from Celanese, Dallas Tex.), 100% solids, weredisintegrated for 5 minutes in a British Disintegrator. The mixture wasdiluted to 4 L deionized water and adjusted to a pH of 7.2-7.4 usingNaHCO₃. The equivalent of 1 g/kg (2 Lb/T) Kymene and 0.13 g/kg (0.26lb/T) of Perform-PC8138 (both available from Hercules, Wilmington, Del.)were added from 1% solutions each, and mixed for 2 minutes. AKD (alkylketene dimmer) available from Hercules, Inc., Wilmington Del. at 2g/kg(4 lb/T) and 4.25 g/kg (8.5 lb/Ton) starch (Sta-Lok 300, available fromTate-Lyle), Decatur Ill.) were each added and the mixture stirred fortwo minutes. A 31.75×31.75 cm forming wire (155 mesh) was placed in thebottom of a Noble & Wood 12″ by 12″ handsheet mold, the slurry pouredinto the sheet mold, diluted to 35 liters with deionized water and mixedwith a plunger. The slurry was then drained, dewatered by using blotterswith even hand pressing until the sheet reached a consistency ofapproximately 20%. The sheet was removed from the screen and blottedfurther to approximately 30% solids. Blotters were placed on each sideof the sample, the sample placed between damp felts and then passedthrough a press at 137.8 kPa (20 psi) to further dewater the sample. Thesolids content at this point was approximately 40%. The resulting sheetwas placed on a drum dryer, (surface temperature of 121° C.), betweentwo dry blotters and allowed to dry for 10 minutes. The sample was theninverted and allowed to dry an additional 10 minutes. The sample wasconditioned in a 50% Relative Humidity room for a minimum of 4 hoursprior to testing.

The foregoing application has been described in conjunction with apreferred embodiment and various alterations and variations thereof. Oneof ordinary skill will be able to substitute equivalents in thedisclosed application without departing from the broad concepts impartedherein. It is therefore intended that the present application be limitedonly by the definition contained in the appended claims.

1. An insulating paperboard comprising: at least one layer of cellulosicfibers, said at least one layer of cellulosic fibers further comprisingsynthetic fibers, said cellulosic fibers being present in an amount from40% to 95% of said at least one layer, said synthetic fibers beingpresent in an amount from 5% to 60% of said at least one layer, saidpaperboard being sufficiently insulating to provide a hot water ΔTacross said paperboard of at least 0.6° C. per 0.1 mm of caliper.
 2. Thecellulosic fibers of claim 1 wherein said cellulosic fibers furthercomprise processed cellulosic fibers.
 3. The processed cellulosic fibersof claim 2 wherein said processed cellulosic fibers are selected fromthe group consisting of chemically processed fibers, chemimechanicallyprocessed fibers, jet dried fibers, flash dried fibers and mixturesthereof.
 4. The fibers of claim 3 wherein the processed fibers aremercerized fibers.
 5. The fibers of claim 3 wherein the processed fibersare CTMP fibers.
 6. The fibers of claim 3 wherein the processed fibersare BCTMP fibers.
 7. The fibers of claim 3 wherein the processed fibersare TMP fibers.
 8. The fibers of claim 3 wherein the processed fibersare jet dried fibers.
 9. The fibers of claim 3 wherein the processedfibers are flash dried fibers.
 10. The synthetic fibers of claim 1wherein the synthetic fibers are polyester fibers.
 11. The syntheticfibers of claim 9 wherein the synthetic fibers are crimped fibers. 12.The insulating paperboard of claim 2, wherein said paperboard has adensity of less than 0.5 g/cc.
 13. The insulating paperboard of claim 2,wherein said paperboard has a basis weight of from 250 gsm to 500 gsm.14. The insulating paperboard of claim 2, wherein the caliper of saidpaperboard is greater than or equal to 0.5 mm.
 15. The insulatingpaperboard of claim 2, wherein said paperboard has a hot water ΔT of atleast 4.5° C. at a caliper of 0.5 mm and a hot water ΔT of 8.7° C. at acaliper of at least 1 mm, said hot water ΔT being a substantially linearprogression relative to caliper in the temperature range from below 4.5°C. to above 8.7° C.
 16. The insulating paperboard of claim 14, whereinsaid linear progression extends from a ΔT of 4.5° C. to a ΔT of 8.7° C.17. The insulating paperboard of claim 1, wherein said paperboard is atleast a two-ply board, said at least one ply containing said cellulosicfibers and synthetic fibers.